The present invention relates to a focusing apparatus of an electron microscope and more particularly to a focusing apparatus of an electron microscope which conducts the correction of astigmatism automatically.
Regarding a usual apparatus of focusing as the preceding stage of correction of astigmatism in the electron microscope wherein the correction of astigmatism is conducted automatically, i.e., a apparatus of determining a circle of least confusion, a description is made in U.S. Pat. No. 4,214,163 according to which the circle of least confusion is determined by the following procedures.
(1) An electron beam is made to scan in the direction X on a sample, a signal inversely corresponding to the radius of the electron beam is obtained in each scan, and an exciting current I.sub.1, for a focusing lens whereby the signal is turned to be maximum is determined.
(2) The electron beam is made to scan in the direction Y on the sample, a signal inversely corresponding to the radius of the electron beam is obtained in the same way as the above, and an exciting current I.sub.2 for the focusing lens whereby the signal is turned to be maximum is determined.
(3) The mean value (I.sub.1 +I.sub.2)/2 is calculated from aforesaid exciting currents I.sub.1 and I.sub.2 for the focusing lens, and the exciting current for the focusing lens is set at his mean value (I.sub.1 +I.sub.2)/2.
By the apparatus described above, the circle of the electron beam focused on the sample becomes the circle of least confusion. The position of this circle of least confusion is equivalent to the position of a focus obtained when the astigmatism is corrected.
In the prior art, as described above, the electron beam is made to scan is each of the directions X and Y, the exciting current for the focusing lens whereby a variation component of a current of a secondary electron or the like generated from the sample on the occasion is turned to be maximum is determined in each scan, and the exciting current for the focusing lens at the time when the circle of least confusion is formed is determined from the mean value of the exciting currents thus determined. The variation component of the secondary electron current is generated by a shape or a pattern on the surface of the sample.
According to this apparatus, however, there occurs a problem that the exciting current for the focusing lens corresponding to the circle of least confusion can not be determined exactly when the shape or pattern on the surface of the sample is non-isotropic as in a small part of IC pattern, for example.
FIG. 3 shows the relationship between the focusing lens exciting current applied when an electron beam is made to scan on the sample in each of the directions X and Y with the current varied and an electron beam radius corresponding signal at this time.
By the way, the electron beam radius corresponding signal means a signal inversely corresponding to the radius of the electron beam and is obtained as a variation component of the current of secondary electrons generated from the sample. The more the electron beam is focused on the sample, the bigger the electron beam radius corresponding signal becomes.
This figure shows particularly a case ((b) of the FIG. 3) wherein the maximum value being an extremely large point is not obtained when scanning is conducted in the direction Y, since the variation component of the shape or pattern on the surface of the sample is small, while the maximum value I.sub.1, being the extremely large point is obtained ((a) of the FIG. 3) when the scanning is conducted in the direction X.
In the case when the shape of the surface on the sample is non-isotropic and therefore the maximum value being the extremely large point does not appear distinctly, as described above, the aforesaid exciting currents I.sub.1 and I.sub.2 can not be determined exactly according to the prior art, and consequently it is very difficult to exactly determine the exciting current for the focusing lens corresponding to the circle of least confusion.
While the apparatus of scanning wherein the electron beam is made to scan in the direction X and Y separately is described in the above-described example, two extremely large points are obtained as shown in FIG. 5(a) even when the electron beam is made to scan circularly, on condition that the shape of the sample is isotropic, and therefore the exciting current for the focusing lens corresponding to the circle of least confusion can be determined exactly by taking the mean value of said points. When the shape of the sample is non-isotropic, however, the two extremely large points can not be obtained as shown in FIG. 5(b) for the focusing lens corresponding to the circle of least confusion can be determined exactly.